In typical mining operations, an earth borehole apparatus is used to drill a borehole and then obtain a core sample from a desired subsurface locations. Analysis of a core sample provides information as to the composition of the subsurface formation and helps geologists determine whether further mining activity is warranted. Apparatus for conducting coring operations, as well as core sampling tools, e.g., core barrels, are well known to those skilled in the art.
In the drilling of a conventional earth and/or hard rock borehole prior to coring, successive lengths of the casing are connected to a suitable bottom-hole assembly, such as a reverse circulation air hammer or a drill bit. In a typical coring operation, the apparatus comprises a mast, a powered drill head or drive unit with a chuck mounted on the mast for longitudinal movement therealong, and threaded tubular pipe similar to surface casing used in oil and gas well drilling, or a rod string.
Typically, a borehole used in the mining exploration coring operations will be up to about 3,000 meters deep, and can be vertical or at an angle up to about 45°. Drilling below a cased surface borehole may be conducted with various types of drilling methods, such as diamond bits or percussion reverse circulation air hammer bits. The cuttings are commonly returned to the surface using an aqueous medium. The fluid is pumped down the drill string and returned up the annulus between the borehole and the drill string. The cuttings and chip samples may be analyzed to determine the general composition of the subsurface formation at any given depth. In the past, coring activities have been conducted after drilling with a string of rods, or with casing pipes extending to the desired depth to determine the precise composition of the cored sample.
One of the drawbacks of these conventional methods of drilling is that it requires the making and breaking of successive length of threaded drill pipe, which is jointed. This operation is time consuming and labor intensive, as well as posing safety concerns. The use of so-called down the hole (DTH) percussive drilling assemblies, particularly so-called water hammers, has found wide-spread acceptance in the mining field for percussive drilling of a bore hole in hard rock condition.
Relevant patents include U.S. Pat. Nos. 4,694,911, 5,476,421, 5,647,445, 5,803,118, 6,125,952, 7,073,610, 7,240,744, 7,617,886, and 7,748,478. Other publications of interest include a description of products and services of Coil Tubing Technology Holding Inc., Products & Services, Operational Strategy by Scientific Prospectus: Integrated Ocean Drilling Program Expedition 313, Steerable Percussion Air Drilling System by Huy Bui, et al., Percussion Drilling in Oil Industry: Review and Rock Failure Modelling by Gang Han, et al., Water Driven Down-the-Hole Well Drilling Equipment for Hard Rock by Bo Nordell, et al., U.S. Publications 2004/0140131 and WO 2008/068546.
The disadvantages of the prior art are overcome by the present invention, an improved system for drilling and coring a mining exploration well is hereinafter disclosed.